Patch from Nick Bart.

   - added more biblatex entry types
   - regrouped code for field mappings in loose categories: author/editor,
   titles, dates, publisher/location, etc.
   - added (probably overly complicated and un-haskell-ish) hack for
   biblatex date and urldate; no support for date ranges yet
   - fixed titles of @periodical
   - various other small fixes